According to a conventionally known method of measuring the thickness of an epitaxial layer (epi-layer) formed on a semiconductor substrate (wafer) in a nondestructive and noncontact manner, the wafer is irradiated with infrared light, and an interference pattern is analyzed that is generated by a difference between an optical path of interface reflecting light caused by a difference in refractive index observed at an interface between the substrate and the epi-layer and an optical path of surface reflecting light.
As an example, patent literature 1 discloses a method of evaluating the thickness of a multilayer film based on a spatialgram obtained by Fourier transforming an interference spectrum from a multilayer film of a sample using a Fourier transform infrared spectroscopy (FT-IR).
The spatialgram contains bursts that are generated by mutual intensification of total light beams caused by interference when a difference in optical path determined by a traveling position of a moving mirror forming the FT-IR agrees with a difference in optical path between all reflection light components from the sample. Patent literature 1 recites that a distance between the bursts corresponds to the difference in optical path between the reflection light components, and discloses a technique of estimating the thickness of the epi-layer by dividing the distance between the bursts by the refractive index of the epi-layer.